U.S. patent application number 13/251415 was filed with the patent office on 2013-04-04 for method of reducing the effect of preheat time variation during shape memory alloy actuation.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS LLC. The applicant listed for this patent is Paul W. ALEXANDER, Alan L. BROWNE, Xiujie GAO, Lei HAO, Guillermo A. HERRERA, Nancy L. JOHNSON, Geoffrey P. MC KNIGHT. Invention is credited to Paul W. ALEXANDER, Alan L. BROWNE, Xiujie GAO, Lei HAO, Guillermo A. HERRERA, Nancy L. JOHNSON, Geoffrey P. MC KNIGHT.
Application Number | 20130081493 13/251415 |
Document ID | / |
Family ID | 47878854 |
Filed Date | 2013-04-04 |
United States Patent
Application |
20130081493 |
Kind Code |
A1 |
GAO; Xiujie ; et
al. |
April 4, 2013 |
METHOD OF REDUCING THE EFFECT OF PREHEAT TIME VARIATION DURING
SHAPE MEMORY ALLOY ACTUATION
Abstract
A system for and method of reducing the effects of preheat
period variation in shape memory alloy actuation, include sensing
the removal of motion delay due to slack, backlash, and/or
compliance in the actuator and drive-train of the system, and
determining actuator activation, as a result thereof.
Inventors: |
GAO; Xiujie; (Troy, MI)
; HAO; Lei; (Troy, MI) ; ALEXANDER; Paul W.;
(Ypsilanti, MI) ; HERRERA; Guillermo A.;
(Winnetka, CA) ; JOHNSON; Nancy L.; (Northville,
MI) ; MC KNIGHT; Geoffrey P.; (Los Angeles, CA)
; BROWNE; Alan L.; (Grosse Pointe, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GAO; Xiujie
HAO; Lei
ALEXANDER; Paul W.
HERRERA; Guillermo A.
JOHNSON; Nancy L.
MC KNIGHT; Geoffrey P.
BROWNE; Alan L. |
Troy
Troy
Ypsilanti
Winnetka
Northville
Los Angeles
Grosse Pointe |
MI
MI
MI
CA
MI
CA
MI |
US
US
US
US
US
US
US |
|
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS
LLC
Detroit
MI
|
Family ID: |
47878854 |
Appl. No.: |
13/251415 |
Filed: |
October 3, 2011 |
Current U.S.
Class: |
74/409 |
Current CPC
Class: |
G05B 2219/41032
20130101; G05B 2219/49206 20130101; Y10T 74/19623 20150115; G05B
19/404 20130101 |
Class at
Publication: |
74/409 |
International
Class: |
F16H 55/18 20060101
F16H055/18 |
Claims
1. A method adapted for implementation by a system comprising a
shape memory alloy actuator and drive-train driven by the actuator,
and presenting a motion delay due to at least one of slack,
backlash, and compliance presented within the system in a
de-actuated state, and for reducing the effects of preheat period
variation when activating the actuator, said method comprising the
steps of: a) exposing the actuator to an activation signal; b)
causing the actuator to start to transform and removing at least a
portion of the motion delay, as a result of causing the actuator to
start to transform; c) sensing the removal of said at least a
portion of the motion delay; and d) autonomously determining
activation of the actuator based on sensing the removal of said at
least a portion of the motion delay.
2. The method as claimed in claim 1, wherein the motion delay is
due to a tolerable degree of slack within the actuator.
3. The method as claimed in claim 2, wherein the actuator comprises
a point, a tab defining a surface is attached to the actuator at
the point, and step c) further comprises the steps of securely
positioning a position sensor relative to the actuator, such that
the sensor is operable to selectively engage the tab and determine
a change in position by the point.
4. The method as claimed in claim 3, wherein the sensor is
photoelectric in operation, and the tab defines a plurality of
through-holes.
5. The method as claimed in claim 3, wherein the sensor includes a
lateral contact operable to physically engage the actuator only
when deactivated, and step c) further comprises the steps of
determining disengagement between the actuator and contact.
6. The method as claimed in claim 3, wherein the sensor is
positioned at a predetermined location, so as to maximize the
observable slack removal.
7. The method as claimed in claim 1, wherein the motion delay is
due to a tolerable degree of compliance within the drive-train.
8. The method as claimed in claim 1, wherein the motion delay is
due to a tolerable degree of backlash in the drive-train.
9. The method as claimed in claim 8, wherein the system is an
active vent system, the drive-train further includes at least one
gear and/or rack cooperatively presenting intermeshed teeth, and
the backlash is presented by spacing between the intermeshed
teeth.
10. The method as claimed in claim 9, wherein the sensor is a
potentiometer operable to detect rotation by a driven one of said
at least one gear.
11. The method as claimed in claim 1, wherein step c) further
comprises the steps of securing a linear position sensor relative
to the drive-train.
12. The method as claimed in claim 11, wherein the sensor is
positioned at a predetermined location proximate the actuator, so
as to maximize the observable removal offered by the
drive-train.
13. The method as claimed in claim 1, wherein the system further
comprises a controller operable to perform an action, and
communicatively coupled to the sensor, said method further
comprising: e) causing the controller to perform the action as a
result of determining the activation of the actuator.
14. The method as claimed in claim 13, wherein the controller is
operable to perform an overload protection action, and step e)
further includes the steps of terminating the signal.
15. The method as claimed in claim 1, further comprising: e)
returning the system to the de-actuated state, and autonomously
regenerating the motion delay, after determining activation.
16. The method as claimed in claim 15, wherein step e) further
includes the steps of engaging the actuator with magnetism, so as
to induce strain.
17. The method as claimed in claim 15, wherein the drive-train
comprises a plurality of racks and/or gears cooperatively
presenting intermeshing teeth, and step e) further includes the
steps of engaging the racks and/or gears with biasing springs
operable to regenerate the motion delay after activation.
18. A method adapted for implementation by a system operable to
produce a performance, and comprising a shape memory alloy actuator
drivenly coupled to a drive-train, wherein the actuator and/or
drive-train presents motion delay in the de-actuated state, and for
improving the performance, said method comprising the steps of: a)
continually exposing the actuator to an activation signal operable
to cause preheat and the start of transformation by the actuator;
b) monitoring the duration of exposure to the signal by the
actuator, and observing the motion delay over time; c) sensing the
start of removal of at least a portion of the motion delay, and
determining activation by the actuator as a result thereof; d)
determining a secondary information, as a result of steps b) and
c); e) adjusting an algorithm, timer, or threshold based on the
secondary information; and f) improving the performance as a result
of adjusting the algorithm, timer, or threshold.
19. The method as claimed in claim 18, wherein the secondary
information is the preheat period of the actuator.
20. The method as claimed in claim 18, wherein the secondary
information is the overall actuation time of the system.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present disclosure generally relates to methods of
reducing the effect of preheat period variation in shape memory
alloy (SMA) actuators, and more particularly, to a system for and
method of doing the same that utilizes the inherent characteristics
of backlash and slack within the system.
[0003] 2. Discussion of Prior Art
[0004] Shape memory alloy actuators vary in preheat period, i.e.,
the time it takes to heat the SMA actuator to just before
activation, as a result of many inherent and external factors,
including ambient temperature differences, the internal temperature
of the actuator (i.e., degree of cooling), constituency differences
from actuator to actuator, the cycle life/usage of the actuator,
and the change in voltage (where activated on-demand) from circuit
to circuit/application to application. Variation in preheat period
presents concerns and challenges for systems operations as a whole,
and more particularly, to software-based peripherals/algorithms
(e.g., overload protection software) that rely upon preheat period
as a trigger or for feedback. To compensate, actuators having large
preheat period tolerances have been implemented; however, these
tolerances present concerns of their own. Among other things, large
tolerances reduce precision, and may result in the ineffectiveness
of the system. In an overload protection algorithm, for example,
imprecision may further result in the failure to timely abate an
overload condition.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention presents a method of reducing the
effect of preheat period variation during shape memory alloy
actuation, which takes advantage of slack, backlash, or compliance
typically inherent in most systems. More particularly, the
inventive system and method utilize the removal of slack, backlash,
or compliance as a more accurate indicator of SMA activation
compared to conventional temporal and signal profile based
measures, and uses this indicator to better predict or control
system performance. As a result, the invention is useful for
providing more accurate and effective software based
peripherals/algorithms without the addition of hardware, which
results in devices that properly function in a broader range of
conditions. Thus, the invention is useful for widening the
application of software based measures, which reduce the number of
moving parts, complexity, and cost of the overall system, in
comparison to mechanical counterparts. Finally, the invention is
further useful for providing novel means for acquiring secondary
information that may be used to enhance performance.
[0006] In general, the invention concerns a method adapted for
implementation by a system comprising a shape memory alloy actuator
and drive-train. The system is configured such that a tolerable
degree of slack, backlash, or compliance is presented in the
actuator and/or drive-train, respectively, when the system is in
the de-actuated state. The method comprises the steps of exposing
the actuator to an activation signal, causing the actuator to
preheat and then activate, so as to remove at least a portion of
the slack, backlash, or compliance, sensing the removal, and
determining start of activation of the actuator based
thereupon.
[0007] In another aspect of the invention, and where activation of
the wire triggers a performance, the method includes continually
exposing the actuator to an activation signal, monitoring the
duration of exposure to the signal by the actuator, sensing the
start of removal of the slack, backlash, or compliance, and
determining the start of transformation by the actuator as a result
of sensing the start of removal, determining secondary information,
such as preheat period, delay due to backlash, or the overall time
to actuation, based on the duration of exposure, and sensing the
completion of removal of the slack and/or backlash. The method
further includes the steps of adjusting an algorithm, timer, or
threshold operable to produce the performance based on the
secondary information. Finally, the performance is improved as a
result of adjusting the algorithm, timer, or threshold.
[0008] The disclosure may be understood more readily by reference
to the following detailed description of the various features of
the disclosure and the examples included therein.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0009] A preferred embodiment(s) of the invention is described in
detail below with reference to the attached drawing figures of
exemplary scale, wherein:
[0010] FIG. 1 is an elevation of a system comprising a shape memory
alloy wire actuator and drive-train comprising a plurality of gears
and racks in de-actuated states, wherein the actuator presents
slack and the drive-train presents backlash (i.e., spacing between
teeth) in the de-actuated state, in accordance with a preferred
embodiment of the invention;
[0011] FIG. 1a is a cross-sectional elevation of the wire actuator
and perforated tab taken along A-A in FIG. 1, wherein the wire and
tab are in the deactivated and activated (hidden line type)
positions, in accordance with a preferred embodiment of the
invention;
[0012] FIG. 1b is a partial elevation of a drive train comprising a
gear and rack defining intermeshed gear and rack teeth, wherein
backlash is reflected as the change in angular position of the
teeth (compare continuous and hidden line type);
[0013] FIG. 1c is a partial elevation of a drive-train comprising
first and second gears presenting intermeshed teeth, wherein the
teeth include magnetic elements that function to space the teeth,
in accordance with a preferred embodiment of the invention; and
[0014] FIG. 2 is an elevation of a system comprising a shape memory
alloy wire actuator and drive-train further including return and
slack regenerating mechanisms, in accordance with a preferred
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The following description of the preferred embodiments is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
[0016] The present invention concerns a system 10 (FIGS. 1-2) for
and method of reducing the effects of preheat period variation in
shape memory alloy actuation to effect a more accurate
determination of activation; and more particularly, to a system 10
and method that uses the slack, backlash (i.e., the quantity of
relative translation amongst driven components necessary to
transfer the drive force from the actuator to the
output/destination), and/or compliance (i.e., the quantity of
elastic deflection, compression, flexure, or otherwise structural
give in the drive components themselves during transfer) inherent
within the system 10 to accomplish the same. That is to say, the
system 10 presents a motion delay in one of the above manners, as
is typically the case. As will be further described below, the
system 10 employs sensory technology to detect the removal of the
slack, backlash, and/or compliance to discern SMA activation, as
well as determine secondary information where desired. The
invention may be employed wherever SMA actuators are utilized, and
over a wide range of applications. In many systems, including an
active vent, for example, the present invention may be used to
improve the accuracy of a software-based overload protection
algorithm, and more particularly, to begin tracking the period of
maximum acceptable exposure, when an output is not detected (e.g.,
the louvers of the vent won't open, etc.), at a time more
temporally corresponding to the actual moment of SMA
activation.
[0017] As used herein, shape memory alloys (SMA's) generally refer
to a group of metallic materials that demonstrate the ability to
return to some previously defined shape or size when subjected to
an appropriate thermal stimulus. Shape memory alloys are capable of
undergoing phase transitions in which their yield strength,
stiffness, dimension and/or shape are altered as a function of
temperature. The term "yield strength" refers to the stress at
which a material exhibits a specified deviation from
proportionality of stress and strain. Generally, in the low
temperature, or martensite phase, shape memory alloys can be
pseudo-plastically deformed and upon exposure to some higher
temperature will transform to an austenite phase, or parent phase,
returning to their shape prior to the deformation.
[0018] Shape memory alloys exist in several different
temperature-dependent phases. The most commonly utilized of these
phases are the so-called Martensite and Austenite phases. In the
following discussion, the martensite phase generally refers to the
more deformable, lower temperature phase whereas the Austenite
phase generally refers to the more rigid, higher temperature phase.
When the shape memory alloy is in the Martensite phase and is
heated, it begins to change into the Austenite phase. The
temperature at which this phenomenon starts is often referred to as
Austenite start temperature (A.sub.s). The temperature at which
this phenomenon is complete is called the Austenite finish
temperature (A.sub.f).
[0019] When the shape memory alloy is in the Austenite phase and is
cooled, it begins to change into the Martensite phase, and the
temperature at which this phenomenon starts is referred to as the
Martensite start temperature (M.sub.s). The temperature at which
Austenite finishes transforming to martensite is called the
Martensite finish temperature (M.sub.f). Generally, the shape
memory alloys are softer and more easily deformable in their
Martensitic phase and are harder, stiffer, and/or more rigid in the
Austenitic phase. In view of the foregoing, a suitable activation
signal for use with shape memory alloys is a thermal activation
signal having a magnitude to cause transformations between the
Martensite and Austenite phases.
[0020] Shape memory alloys can exhibit a one-way shape memory
effect, an intrinsic two-way effect, or an extrinsic two-way shape
memory effect depending on the alloy composition and processing
history. Annealed shape memory alloys typically only exhibit the
one-way shape memory effect. Sufficient heating subsequent to
low-temperature deformation of the shape memory material will
induce the Martensite to Austenite type transition, and the
material will recover the original, annealed shape. Hence, one-way
shape memory effects are only observed upon heating. Active
materials comprising shape memory alloy compositions that exhibit
one-way memory effects do not automatically reform, and will likely
require an external mechanical force if it is judged that there is
a need to reset the device.
[0021] Intrinsic and extrinsic two-way shape memory materials are
characterized by a shape transition both upon heating from the
martensite phase to the Austenite phase, as well as an additional
shape transition upon cooling from the Austenite phase back to the
Martensite phase. Active materials that exhibit an intrinsic shape
memory effect are fabricated from a shape memory alloy composition
that will cause the active materials to automatically reform
themselves as a result of the above noted phase transformations.
Intrinsic two-way shape memory behavior must be induced in the
shape memory material through processing. Such procedures include
extreme deformation of the material while in the martensite phase,
heating-cooling under constraint or load, or surface modification
such as laser annealing, polishing, or shot-peening. Once the
material has been trained to exhibit the two-way shape memory
effect, the shape change between the low and high temperature
states is generally reversible and persists through a high number
of thermal cycles. In contrast, active materials that exhibit the
extrinsic two-way shape memory effects are composite or
multi-component materials that combine a shape memory alloy
composition that exhibits a one-way effect with another element
that provides a restoring force to reform the original shape.
[0022] The temperature at which the shape memory alloy remembers
its high temperature form when heated can be adjusted by slight
changes in the composition of the alloy and through heat treatment.
In nickel-titanium shape memory alloys, for instance, it can be
changed from above about 100.degree. C. to below about -100.degree.
C. The shape recovery process occurs over a range of just a few
degrees and the start or finish of the transformation can be
controlled to within a degree or two depending on the desired
application and alloy composition. The mechanical properties of the
shape memory alloy vary greatly over the temperature range spanning
their transformation, typically providing the system with shape
memory effects, superelastic effects, and high damping
capacity.
[0023] Suitable shape memory alloy materials include, without
limitation, nickel-titanium based alloys, indium-titanium based
alloys, nickel-aluminum based alloys, nickel-gallium based alloys,
copper based alloys (e.g., copper-zinc alloys, copper-aluminum
alloys, copper-gold, and copper-tin alloys), gold-cadmium based
alloys, silver-cadmium based alloys, indium-cadmium based alloys,
manganese-copper based alloys, iron-platinum based alloys,
iron-platinum based alloys, iron-palladium based alloys, and the
like. The alloys can be binary, ternary, or any higher order so
long as the alloy composition exhibits a shape memory effect, e.g.,
change in shape orientation, damping capacity, and the like.
[0024] It is appreciated that SMA's exhibit a modulus increase of
2.5 times and a dimensional change (recovery of pseudo-plastic
deformation induced when in the Martensitic phase) of up to 8%
(depending on the amount of pre-strain) when heated above their
Martensite to Austenite phase transition temperature. It is
appreciated that thermally induced SMA phase changes are one-way so
that a biasing force return mechanism (such as a spring) would be
required to return the SMA to its starting configuration once the
applied field is removed. Joule heating can be used to make the
entire system electronically controllable.
[0025] Returning to the configuration and steps of the present
invention, FIG. 1 shows an exemplary system 10 comprising a shape
memory alloy wire actuator 12 and drive-train 14; together the
actuator 12 and drive-train 14 present a drive. The term "wire", as
used herein, is non-limiting, and encompasses other equivalent
geometric configurations such as bundles, braids, cables, ropes,
chains, strips, etc. Moreover, it is appreciated that the actuator
12 may present other configurations, such as SMA springs, sheet,
torque tubes, etc. As previously mentioned, the system 10 functions
to detect removal of at least a portion of the slack, backlash,
and/or compliance and correlate the detection with the commencement
of activation. In a first aspect of the invention, the actuator 12
is configured so as to present slack (i.e., a bowed, sinuous, or
curved profile) when deactivated; and removal of the slack is used
to determine when the actuator is activated. More particularly,
where actuation is desired, the inventive method begins at a first
step by heating the SMA wire 12 (e.g., through Joule heating) over
a preheat period. That is to say, the wire 12 is continually
exposed to an activation signal over an indeterminable preheat
period by a suitable signal source 16 (FIG. 1).
[0026] Despite the indeterminable heating period, a generally
accurate time of activation is detected by physically sensing the
removal of the slack at a second step. As such, it is appreciated
that an external sensor 18 further composes the system 10. For
example, a position sensor 18, such as a photoelectric sensor, may
be used to detect a change in position by a reflective surface 20a
defined by a tab 20 fixedly attached to the wire 12 (FIGS. 1 and
1a), or by the wire 12 itself. As shown in FIG. 1a, the tab 20 may
define a plurality of through-holes 22, and extend orthogonal to
the wire 12, where a photoelectric sensor 18 is oriented and
positioned to register either an "ON" or "OFF" value, dependent
upon alignment of its light source 21 with a through-hole 22. When
the wire 12 is caused to undergo transformation and begins to
contract, it initially removes the slack, irrespective of system
output conditions. This causes the tab 20 to laterally translate,
and the sensor 18 to toggle "ON" and "OFF" values as the light
source 21 intermittently encounters through-holes 22. The change in
values registers a detected change in surface position that is
correlated to slack removal. Thus, it is appreciated that the tab
20 is preferably attached to the point (e.g., a vertex of the
curved profile) or section of the wire 12 that undergoes the most
lateral displacement, so as to maximize the observable slack
removal.
[0027] More preferably, maximum displacement is ensured and slack
may be produced by magnetizing the tab 20 and causing it to
laterally engage adjacent ferrous material 24 (FIG. la). That is to
say, in a preferred embodiment, the tab 20 may further function to
produce the slack itself by laterally straining the wire 12 in its
Martensitic phase (e.g., via gravity, magnetism, etc.). The wire 12
is configured such that actuation overcomes this effect with
minimal hindrance.
[0028] Alternatively, disengagement between the tab 20 and adjacent
material 24 may be sensed directly. That is to say, the adjacent
material 24 may function as a contact that is closed when engaged
with the tab 20, and opened when disengaged. Once activation
through slack removal is determined, the method proceeds to the
next step where the system 10 is configured to trigger or provide
feedback to the system 10 in order to perform an action. In the
previous example, the system 10 may be further configured to
trigger an overload protection routine that terminates the
activation signal if a threshold period of exposure is surpassed
without achieving the desired output. Thus, the preferred system 10
further includes a controller 26 communicatively coupled to the
actuator 12, signal source 16, and sensor 18.
[0029] At a final step, the preferred system 10 is configured to
autonomously return the output, and regenerate a tolerable degree
slack within the actuator 12 for subsequent use. To that end, the
tab 20 may be attracted by the adjacent magnetic material 24 when
the SMA is in its deactivated state, so as to stretch the wire 12.
In another example shown in FIG. 2, an extension spring 28
presenting a spring modulus, k.sub.1, is drivenly coupled to a
sliding drive-train 14 antagonistic to the actuator 12; the
drive-train 14 includes inner and outer telescoping parts 30,32,
with the actuator 12 being coupled to the inner part 30 and the
spring 28 coupled to the outer part 32 (FIG. 2); and a compression
spring 34, having a spring modulus k.sub.2<k.sub.1,
intermediately engages the parts 30,32, so as to drive them towards
the mated condition shown in FIG. 2. Once the actuator 12 is
deactivated and caused to revert back to its Martensite phase, the
extension spring 28 works to pseudoplastically strain the wire 12
and return the drive-train 14 to its original position. To
autonomously regenerate slack, the drive-train 14 further include
at least one, and more preferably a plurality of pawls 36 having
protracted arms 38 (FIG. 2). The drive-train 14 is configured such
that the pawls 36 are caused to rotate as the parts 30,32 are
returned to the original position; and to that end, includes a
transmission (not shown) operable to convert the linear motion into
rotational displacement. As the arms 38 engage the inner part 30,
driving it into the outer part 32 against the action of the
compression spring 34, the wire 12 is further strained. Once a half
revolution is complete, the parts 30,32 snap back to the mated
condition, thereby producing slack in the wire 12. As previously
mentioned, a weight or magnetism may be employed to further produce
slack within the wire 12.
[0030] In a second aspect of the invention, the drive-train 14 may
be engineered to provide a tolerable degree of backlash in the
system 10, in addition to or lieu of slack in the actuator 12. That
is to say, in this configuration, the wire 12 may be strained taut,
as is typically desired to effect more rapid response during
actuation. Returning to FIGS. 1 and lb, where the drive-train 14
includes a plurality of gears 40 and racks 42 presenting
intermeshed teeth 40a,42a, the teeth 40a,42a may be spaced to
produce a suitable degree of backlash (i.e., play or give before
the subsequent component is driven). As in the previous method, a
position sensor 18 is secured relative to the drive-train 14, and
operable to detect the beginning of the removal of backlash. In the
illustrated embodiment, a rotary sensor 18, such as a
potentiometer, is positioned relative to the first driven or
control gear 40, so as to increase the observable backlash
removal--it is appreciated that backlash amongst subsequent
components within the drive-train 14 is reflected by increasing
displacement in a given gear 40 or rack 42 as it drives the
subsequent components. It is further appreciated that increased
proximity to the actuator 12 results in faster backlash removal
detection, as well as an increased amount of backlash removal to
detect.
[0031] The preferred system 10 further comprises a controller 26
communicatively coupled to the position sensor 18 (also shown in
FIG. 1). The controller 26 is operable to perform an action or
performance (e.g., execute an overload protection algorithm), once
activation of the SMA wire 12 is detected by sensing the removal of
backlash. Detection of SMA activation may be used to predict a
target output, and where feedback is provided relating to the
output, discern a failure (e.g., blockage of the target output).
The preferred method includes returning the drive-train 14 to the
de-actuated state, and autonomously regenerating a tolerable degree
of backlash in the drive-train 14 for future use. For example, in
this configuration, it is appreciated that torsion springs 28,
coaxially aligned with each gear 40, may be used to reset the
system 10. Equally, extension springs (not shown) may be drivenly
coupled to each rack 42, so as to present a biasing force
antagonistic to the actuator 12. Alternatively, magnetism may be
used; this time, by repelling intermeshed teeth 40a,42a, wherein
adjacent teeth surfaces ahead of actuation comprise magnetic
elements 44 of like poles, and/or attracting adjacent surfaces
arrear actuation comprise elements 44 of opposite poles (FIG.
1c).
[0032] As previously mentioned, a third aspect of the invention
involves measuring compliance within the drive-train 14, such as
compression amongst gear teeth, etc., or flexure/bending in axel
rods, racks, lever arms, etc. In addition to material composition,
it is appreciated that the geometric shape of drive components play
a significant role in the amount of compliance presented; for
example, the more elongated a component, the more likely that
compliance in the form of flexure will be generated. Here, the
sensor 18, such as a linear position sensor, is preferably
positioned at or near the actuator 12, so as to be able to detect
the aggregate compliance in the system 10. Again, the total
compliance offered by the drive-train 14 must be tolerable, so as
not to measurably impact the effective stroke of the actuator
12.
[0033] Lastly, it is appreciated that removal of slack, backlash,
and/or compliance may also be used to provide secondary
information, which could then be used to improve system
performance. For example, in addition to discerning actual SMA
activation, the preheat period, delay attributed to
slack/backlash/compliance, and the overall time to actuation (i.e.,
preheat period plus delay) may also be determined by monitoring the
duration of exposure to the signal by the actuator 12 and observing
the slack/backlash/compliance removal over time. The preheat
period, delay, and/or overall time to actuation may then be used,
for example, to adjust an algorithm, timer, or threshold, so as to
tune the system 10 for a given set of conditions. That is to say,
control software may be programmably configured to adjust a
variable to achieve consistent actuation times from the time the
actuation signal is received to the time the device is completely
actuated.
[0034] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal languages
of the claims.
[0035] Also, as used herein, the terms "first", "second", and the
like do not denote any order or importance, but rather are used to
distinguish one element from another, and the terms "the", "a", and
"an" do not denote a limitation of quantity, but rather denote the
presence of at least one of the referenced item. All ranges
directed to the same quantity of a given component or measurement
is inclusive of the endpoints and independently combinable.
* * * * *